I am trying to find affordable art resources for publishing. Basically we can pay for a minimal amount of art, but can't afford enough to fill most of our books. Are there online art depots with free or affordable public domain art, or art that you can buy at a low rate?

Well the first thing you might want to do is determine what projects you might have that could serve with using the most basic art and modern pictures in the project. For example getting gimp downloaded on your comp and modding some public domain images or some purchased ones from http://www.istockphoto.com/?gclid=CImRrNDK_poCFQxM5QodRlAQew for example may be your best option. As you start to earn money from the merch you made, and the book you wrote start looking into buying some more professional work from actual artists. Would you mind indulging on your projects a bit more so that we could get a better idea of where you might be able to find pictures along the same themes?

flickr is a photography site where anyone can register for an account and upload pictures for other people to view. It's useful as an art resource because people can easily specify a Creative Commons license for their pictures. CC-BY is the same as "Creative Commons Attribution only", i.e. you can use the photo however you like as long as you credit the photographer. Even if a photo isn't explicitly licensed for re-use, you can contact the user and ask them about it.

If you're interested in more Creative Commons works, Google and Yahoo both have options in their Advanced Search pages to restrict the source to Creative Commons-licensed pages.

BibliOddyssey has scans of art from old manuscripts; many of their images are available (via flickr) under CC-BY. Not sure how much use they'll be for your modern game, but an amazing resource.

Another source, illodelli, got mentioned in a thread on the Connections subforum. It's a site where some artists got together and put up spare pieces they had lying around for one-use non-exclusive licensing at low prices.